net: Add Intel Topcliff GMAC driver

Add a new driver for the Gigabit Ethernet MAC found on Intel Topcliff
Platform Controller Hub. Tested under 10/100 half/full duplex and 1000
full duplex modes using ping and tftpboot commands.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>